1. Field of Invention
This invention relates generally to a support system for continually providing controlled body weight support during running, walking or standing, and in particular, to a support system for use in locomotor training.
2. Related Art
There is a need to create an environment where the amount of force on the lower body is reduced during walking, running or standing, for applications such as athletic training, orthopaedic and neurologic rehabilitation, virtual reality studies and general scientific investigations. This type of environment has been used in studies of astronaut maneuverability during space travel, in gait rehabilitation for neurologic and orthopaedic disorders—where the amount of load on the patients lower body is reduced to provide the subjects with enough support to allow them to stand and step with minimal outside assistance—and in basic science studies to understand how load bearing and gravity affect the mechanical and neurological systems of biological beings.
Several different methods have been used to reduce body weight load during gait. For example, one method includes an underwater treadmill and an adjustable ballasting harness to examine peak ground reaction forces, stride frequency and energy requirements during gait in reduced gravities (20). This water immersion technique has also been used to evaluate the ability of astronauts to perform several tasks in reduced gravity (22). The use of water immersion can be limited by factors including the need to supply a breathing apparatus to subjects, the effects of drag caused by water and the need for special equipment to function underwater.
Another method of creating reduced gravity is to have an aircraft fly a Keplerian trajectory (19). By repeating this flying maneuver and altering the parameters of the trajectory, uniform reduced gravity levels between 0 and 1 gravity, can be obtained for periods up to 30 seconds. This method has been successfully used to measure the pendulum like exchange of potential and kinetic energy during gait at different gravities (3,4). Parabolic flight has an advantage over most reduced gravity simulation techniques in that it places a body force over every particle within a body. In addition, subjects can move with six degrees of freedom and are not restrained by a framework or by a liquid-imposed drag. However the usefulness of parabolic flight is limited due to the very short periods of reduced gravity that are created, and the difficulty, expense and expertise needed to create this environment.
An overhead suspension system is a very popular, inexpensive and simple alternative to water immersion or parabolic flight for the creation of a reduced gravity environment. The suspension system can be as complex as having a subject walking in a supine or horizontal position on a treadmill, with support cables that attach to the trunk, head and individual limb segment and run to an over head counter weight system (6,16), to as simple as having a bicycle saddle attached to a series of stretched springs that when straddled during upright treadmill walking, provides an upward force to the subject (15).
The stretched spring suspension method has been used to study several principles of human gait in reduced gravity including; leg stiffness (15), energetics (10), walk-run transition speeds (17), dynamic similarity (9), gravitational and inertial forces on ground reaction force, mechanical energy fluctuations and exchanges (13) and application of horizontal force on ground reaction force (5). However using a stretched spring as a means of support during walking is limited because of the inherent movement of a person's center of mass during locomotion. Errors in the stretched spring system are attributed to stretching of the spring during gait and frictional forces caused by rubbing motions of the cable.
Several other groups have used variations of upright suspension body weight support systems to study the control of locomotion in humans and to provide locomotor training, a new rehabilitation approach to the recovery of walking after neurologic and orthopaedic injury (7,23).
In locomotor training, a spinal cord injured subject is suspended in a harness above a treadmill, and manual assistance is provided to move the subject's legs in a walking pattern. The key characteristics of this technique are partial unloading of the limbs and assistance of leg movements during stepping on the treadmill. The goal of this technique is to enhance residual locomotor control circuitry that resides in the spinal cord (2).
Many of the subjects undergoing locomotor training are unable to support their entire bodyweight on their own. Because of this, the role of the body weight support device in motor control studies and in locomotor training is to provide subjects with enough support to allow them to stand and step with minimal outside assistance.
In one body weight support system used for locomotor training, the subject wears a harness, similar to a parachute harness, to support the subject vertically in a standing position (1). The lift to the harness is provided by an overhead electric motor using a gearbox and a slip gear manually controlled by an operator. A similar body weight support system uses an overhead motorized lift to support spinal cord injured subjects during stepping on a treadmill with manual assistance (14). Due to the inherent movement of a subject's center of mass, the amount of support force provided to the subject in this very stiff system will vary widely based on the position of subject. For example, because of the stiff position control of the system, during walking, when a subject's center of mass is at its greatest vertical amplitude at midstance, the amount of support provided by the system is decreased in comparison to the amount of support occurring at heel strike, when the subject's vertical center of mass is the lowest.
In spite of these limitations of the overhead electric motor system, several key principles of gait control in spinal cord injured subjects were discovered in studies that utilized this system. First, for spinal cord injured subjects stepping with bodyweight support and manual assistance, mean electromyography (EMG) amplitudes in the soleus, medial grastrocnemius and tibialis anterior are directly related to the peak load during each step (14). Second, spinal cord injured subjects stepping with a similar set up had higher mean EMG amplitudes as treadmill speed increased (21). These results indicate that sensory information from both kinetic and kinematic factors, including movement of the center of mass and ground reaction forces similar to that during normal locomotion, are vital in creating appropriate motor outputs.
Counterweights are another method used to provide a lift to a subject via a support harness during locomotor training. This type of counterweight bodyweight support system was employed to study the relationship between EMG activity in spinal cord injured subjects during stepping, and the level of their lesion (8). The counterweight system is very limited in its ability to provide a constant support force because, as a subject's center of mass moves during walking, the inertial factors of a counter weight tethered to a moving object will cause fluctuations in the amount of support the subject is receiving, thus affecting both center of mass movement and ground reaction forces.
One way to allow center of mass movement during the unloading of body weight while stepping is to use a pneumatic cylinder with a support harness to provide lift to a subject (11). Results suggest that the mean ground reaction force is reduced comparably to the level of body weight support. However, the manner in which the ground reaction force is reduced is not consistent across the force time curve because there is no feedback control. The result is abnormal center of mass movement and ground reaction forces.
A detailed description of a pneumatic system whose aim is to provide a constant support lift to the subject has been presented by Gazzani (12). This system has an error in support force of less than 10% and in general, for the parameters of use (level of support and speed of walking) encountered in clinical practices, the error is less than 5%. The pneumatic system of Gazzani is an improvement over other systems because of its closed looped nature. However, the system is limited because the system is closed by taking solely measurements of pressure inside the pneumatic cylinder and does not obtain measurements of forces exerted on the individual. Further, the characteristics of these pressures do not necessarily represent the effect on the ground reaction forces exerted on the individual, which are ultimately the values that need to be maintained at normative values for effective locomotor training interventions.
Fluctuations in the level of body weight support force in all the upright overhead body weight support systems would seem to be caused primarily by vertical movements of the subject during locomotion. The following equation accounts for the error in support force seen in the pneumatic system of Gazzani:Error=±F+K*X+B*V+M*Awhere F is pure friction, K is stiffness, B is viscous drag, M is moving masses, X is vertical position, V is vertical velocity and A is vertical acceleration (12).
Controlling fluctuations in the level of support force in relation to the effect on the ground reaction forces are of central importance for basic science applications to the understanding of human locomotion and for locomotor training following neurologic disorders or orthopaedic injury. For neurologically or orthopedically impaired patients to safely use locomotor training as a form of rehabilitation, it is essential that the amount of loading they experience is both known and controllable. Moreover, several animal and human studies on the control of locomotion suggest a significant level of recovery can occur, even when supraspinal input is severely compromised or absent, by providing appropriate sensory information related to locomotion. One very important piece of sensory information is limb loading during stepping (7,14). The level of bodyweight support force exerted on a patient during locomotor training will directly influence the amplitude and manner in which the limb is loaded. The amount and pattern of support force through out the gait cycle, will affect the amplitude and pattern of the ground reaction forces during stance and thus should be controlled to represent those observed during over ground walking.